Conventionally, there has been known a developing device configured to develop a latent image formed over a latent image bearer by a developing unit. For example, there is a two-component-type developing device that uses as a developer, a two-component developer containing a toner and a carrier to develop a latent image formed over a latent image bearer. In the two-component-type developing device, a portion of a surface of a developing sleeve constituting a developer bearer and a portion of a surface of an image bearer face each other and form a developing region. Under a magnetic field of a magnetic field generating unit provided in the developing sleeve, a magnetic brush is formed over the developing sleeve and brought close to or into contact with the latent image bearer in the developing region, to thereby attach the toner to the latent image over the surface of the latent image bearer and develop the latent image to a visible image.
In the developing device of this kind, the toner moves from the developing sleeve to the latent image bearer by the effect of a potential difference between the surface potential of the developing sleeve to which a developing voltage is applied and the surface potential of the latent image bearer. As the scheme for applying a developing voltage to the developing sleeve, there are known a scheme of applying a voltage containing only a direct-current component (hereinafter referred to as “DC bias development”), and a scheme of applying a voltage containing an alternating-current component (hereinafter referred to as “AC bias development”) (see, e.g., PTL 1). Meanwhile, in recent years, there has been an acceleration in the changeover from monochrome to full-color in the electrophotographic copier or printer technologies, and the full-color market is in the expanding tendency. In image formation by such an electrophotographic full-color system, it is necessary to keep the amount of a toner over the latent image bearer faithful to the electrostatic latent image, in order to obtain a clear full-color image excellent in color reproducibility.